Understanding individual differences in emotional processing is critical for elucidating the role of emotions in normal and psychopathological functioning in humans. In both humans and primates, individual differences in patterns of asymmetry in anterior brain activity (measured by electroencephalography, EEG) are stable over time and predict aspects of an individual's emotional dispositions (affective style). In humans startle blink responses are reliably modulated based on their emotional state potentiated when negative and attenuated when positive. The amygdala likely plays an important role because it is an integral component of effective responses and an important relay along cortical-limbic pathways. The relationship between frontal cortex asymmetry, negative and positive emotional responsiveness as measured by emotion-modulated startle (EMS), and affective style or temperament will be examined in humans and nonhuman primates. Experiments 1&2 will greatly expand on previous research in humans showing a relationship between asymmetry of frontal cortex EEG activity, negative (anticipatory fear) EMS and positive (anticipatory reward) EMS. A parallel nonhuman primate model of these relationships with rhesus monkeys will also be established. Experiment 3 will investigate the role of affective style in both humans (with self-report scales) and monkeys (with a paradigm measuring fear-related temperament). Finally, the role of the amygdala underlying these variables will be examined through functional brain imaging in humans and lesion studies in rhesus monkeys taking advantage of data from ongoing studies.